This invention relates to an active matrix liquid crystal display device comprising on a substrate an array of switching devices, at least one set of address lines connected to the switching devices, an insulating layer extending over the substrate and covering the address lines, and an array of display pixel electrodes comprising a conductive layer carried on the insulating layer, each of the pixel electrodes being connected to a respective switching device through a contact hole formed in the insulating layer.
An example of such a display device is described in EP-A-0617310. In this device, a row and column matrix array of display pixels is provided, each of which is driven via an associated switching device in the form of a TFT (thin film transistor). As is usual, the device comprises a layer of liquid crystal material disposed between a pair of spaced substrates carrying electrodes which define individual display pixels. The TFTs are carried on the surface of a first substrate together with sets of row, (scanning), conductors and column, (data), conductors through which the TFTs are addressed for driving the display pixels. As in conventional active matrix LCDs using TFTs, each TFT is disposed adjacent the intersection between respective ones of the row and column conductors. The gates of all the TFTs associated with a row of display pixels are connected to a respective row conductor conductor and the sources of all the TFTs associated with a column of pixels are connected to a respective column conductor. Unlike conventional active matrix LCDs, however, in which the individual pixel electrodes are arranged substantially co-planar with, and laterally of, the TFTs, the reflective metal pixel electrodes in this device are carried on an insulating film which extends over the first substrate and covers the TFTs and the sets of address conductors so that the pixel electrodes are positioned generally above the level of the TFTs and the address conductors. Each individual pixel electrode is connected to the drain electrode of its associated TFT through a respective opening formed in the insulating film directly over the drain-electrode. An advantage of this type of construction, in which the array of pixel electrodes and the array of TFTs are provided at respective different levels above the substrate surface, is that the pixel electrodes can be enlarged such that at two opposing sides they extend slightly over adjacent row conductors and at their two other opposing sides they extend slightly over adjacent column conductors rather than being sized smaller than the spacing between adjacent row conductors and adjacent column conductors, and with gaps provided between each edge of the pixel electrode and the adjacent conductor, as in conventional display device arrangements. In this way, therefore, the pixel aperture is increased and in operation more light which passes through the liquid crystal layer and reaches the pixel electrode is reflected back to produce a brighter display output. Moreover, parts of a deposited metal layer which is patterned to form the reflective pixel electrodes can be left immediately overlying the TFTs during the patterning process so as to act a slight shields for the TFTs to reduce photoelectric effects in the TFTs due to light incident thereon, thereby avoiding the need to provide black matrix material on the other substrate for this purpose as is usual. This other, transparent, substrate carries a continuous transparent electrode common to all pixels in the array and, in the case of a colour display, an array of colour filter elements corresponding to the array of pixels with each filter element overlying a respective pixel electrode.
It is known also to integrate drive circuits for driving the display pixels on the same substrate as the switching devices and address lines peripherally of the display array which circuits employ switching devices and connection lines fabricated simultaneously with the switching devices, e.g. TFTs, and address lines from common deposited layers. Such integration avoids the need to provide separately fabricated drive circuits and to connect those circuits to the address lines. In the case of a TFT display device, both the row (scanning) drive circuit and the column, (data) drive circuit can readily be integrated using polysilicon technology although sometimes amorphous silicon technology can be used. Examples of LC display devices using integrated drive circuits are described in the paper entitled "Fully Integrated Poly-Si TFT CMOS Drivers for Self-Scanned Light Valve" by Y. Nishihara et al in SID 92 Digest, pages 609-612, and in the paper entitled "A 1.8-in Poly-Si TFT--LCD for HDTV Projectors with a 5-V Fully Integrated Driver" by S. Higashi et al in SID 95 Digest, pages 81 to 84.
Typically the row (scanning) drive circuit comprises a digital shift register circuit and the column, data, drive circuit comprises a multiplexing circuit. Both circuits utilise conductor lines in the form of bus lines, bus bars, or other conductors carrying for, example, predetermined voltages, such as the Vss and Vdd power supply lines, or signals, for example, clock or video signal lines, which are formed usually from a metallisation used for providing either the set of row address conductors or the set of column address conductors in the pixel array and photolithographically defined at the same time as that set of conductors from the deposited metal layer. Consequently, the thickness of these conductor lines corresponds to the thickness of the address lines and desirably this thickness is maintained as small as possible so as to avoid introducing high steps. Also, because space at the periphery of the substrate is normally at a premium to provide a compact display device, the conductor lines tend to be narrow in width. Problems are encountered, however, as a result of limitations in the electrical conductivity of these lines, and especially their inherent resistance, which can have a significant effect on applied voltages or signals along their length, bearing in mind that they can extend for at least a substantial proportion of the width or height of the display pixel array.